The present invention relates to a modular installation for liquid/gas separation that is of the vertical type, and also to a method of separating a multiphase fluid, in particular oil-water and gas contained in crude oil.
More particularly, the technical field of the invention is the field of oil production, and more particularly from oilfields at sea in great depths.
Deep sea oil production is generally performed from a floating support anchored in the proximity of oil wells that are situated at the sea bottom, i.e. at depths lying in the range 1000 meters (m) to 2500 m, or even more. In general, the floating support has anchor means enabling it to remain in position in spite of the effects of currents, winds, and swell. It also generally includes means for storing and processing oil together with off-loading means for off-loading oil to tankers that call at regular intervals in order to remove the production. The common term for such supports is floating production storage and off-loading supports, and they are referred to throughout the description below by the initials FPSO.
The wellheads are generally connected to said FPSO by undersea pipes, either of the suspended catenary riser (SCR) type, or of the hybrid tower type, comprising:                a vertical riser having its bottom end anchored to the sea bottom and connected to a said pipe resting on the sea bottom, and its top end tensioned by a float immersed in the subsurface to which it is connected; and        a connection pipe, generally a flexible connection pipe, between the top end of said riser and a floating support on the surface, said flexible connection pipe taking on, where appropriate, a dipping catenary curve shape as a result of its own weight, i.e. dipping down well below the float before subsequently rising up to said floating support.        
The entire production of crude oil is thus generally raised on board the FPSO in order to be processed so as to separate the oil proper from the water, the gas, and the sandy components, if any. The oil, once separated, is thus stored on board, the gas is washed and then sent to gas turbines for producing the electricity and the heat needed on board, and then any surplus is injected into the reservoir of the oil field so as to restore pressure in said reservoir. The water, after being released from sand in suspension, is finally either rejected into the sea after thorough extraction of oil particles, or else it is likewise reinjected into the reservoir, with additional seawater taken from the subsurface generally also being added in order to achieve the necessary flow rate for injecting water into the reservoir. The extracted sand, which represents only minimal quantities, is finally washed and then rejected into the sea.
A known method of separating gas, water, and oil from crude oil that is commonly employed on stationary installations on land and on board the FPSOs consists in using enclosures that withstand pressure, that are of very large volume, generally in the form of elongate cylinders, with the crude oil entering via one end and travelling along said enclosure for a duration of about three minutes (min) to about 10 min, during which the various phases separate naturally under gravity before reaching the second end. Gas is then recovered from the top portion of said enclosure, water and sand from the bottom portion, and oil from an intermediate portion. There exists a very large variety of separators of that type, that generally include additional internal devices, such as horizontal, vertical, or sloping screens for the purposes of facilitating separation of the phases and of preventing them from re-mixing at a later stage.
Those separators operate at low pressure, e.g. in the range 3 bars to 10 bars, and sometimes even at negative pressures, in order to optimize degassing the crude oil. If it is desired to install that type of separator at the sea bottom, said enclosure must be capable of withstanding crushing as a result of the pressure that is equal substantially to 100 bars, i.e. substantially 10 megapascals (MPa), for each 1000 m of water depth, and also the internal pressure, which depends on the pressure of the oil reservoir and which can reach 1400 bars in certain circumstances. As a result, transposing such an enclosure to enable it to be used at great or very great depth would require wall thicknesses in the range 100 millimeters (mm) to 300 mm in order to withstand implosion, and such sheet metal elements would be very difficult and very expensive to make and install on the sea bottom at great depth.
Document FR 2 915 403 in the name of the Applicant discloses an undersea gas-liquid separator device, however that device comprises an enclosure containing a set of enclosures arranged vertically that are permanently connected to one another and that are of large weights and volumes, that assembly being found, in certain circumstances, difficult to put into place on the sea bottom and requiring installation ships having considerable offshore hoisting capacity, and in particular requiring the use of installation ships that are longer than 200 m. Also, elements internal to the enclosure, such as sensors, run the risk of failing and/or becoming clogged, and they possibly also run the risk of plugging liquid discharge outlets. Such incidents require maintenance that cannot be performed by disconnecting and recovering the enclosures individually using conventional maintenance vessels (known as “inspection, maintenance, and repair (IMR) vessels”), which are generally of relatively small size and hoisting capacity.
In another field, in document FR 2 961 712 in the name of the Applicant, proposals are made for a modular undersea installation for liquid-liquid separation that is constituted by a plurality of pipes that slope a little relative to the horizontal, that are in parallel, and that are individually connectable and disconnectable at will, and that can therefore be installed at the bottom and thus handled while limiting the weight of the elements that are to be handled from the surface.